Sorry your games were so disappointing. A few thoughts from a necromunda player.

-I would go the opposite way and suggest LOTS more terrain and a smaller battlefield (try 3x3). Your Guardsmen are not going to win a shooting war so they need to get close enough to get enough las-guns on target to take down some enemies and them finish them off in H2H.

-I agree that 6 is too small. Drop some of that gear and get more bodies on the table. Also/alternatively take advantage of the fact that guard kill teams can have 3 heavies (most everyone else only gets 2) and get a couple of grenade launchers out there!

-You're right that shooting at a downed fighter is not always the best decision. It's better to run up and engage them in H2H combat when they're down which is almost alaways fatal. This is much easier when you have lots of terrain on a smaller table making it easier.

-The ladders rule can be problematic depending on your terrain collection. I think even some folks with the new GW terrain have found they need more ladders. Some possibilities might include House ruling the locations of elevators, making up climbing rules or somesuch. I'm planning on buildnig some stand-alone ladders permanemently attached to small bits of scatter terrain that can be placed next to terrain pieces that don't have them. "Promethium Forge" sells ladder packs that might be usefull.

Anywho, I've not played Shadow War yet, so take my suggstions with a grain of salt. However, your issues with the game pretty closely match many of the issues that Necromunda also has and the fixes above might give you a much more enjoyable game. Good Luck!

Beware though, building terrain for Necromunda/ShadowWar is a major rabit hole a 3x3 necromunda table will swallow a regular 8x4 table worth of 40k terrain and ask for more. And that's before you add all the necessary gantrys and walkways.